Governments will no longer be able ignore the impending climate and ecological crisis, Greta Thunberg, the teenage climate activist, has told Extinction Rebellion protesters gathered at Marble Arch in London.

In a speech on Sunday night where she took aim at politicians who have for too long been able to satisfy demands for action with “beautiful words and promises”, the Swedish 16-year-old said humanity was sitting at a crossroads, but that those gathered had chosen which path they wish to take.

Q&A

What is Extinction Rebellion?

Extinction Rebellion is an international protest group that uses non-violent civil disobedience to campaign on environmental issues.

The group says climate breakdown threatens all life on Earth, and so it is rebelling against politicians who 'have failed us', to provoke radical change that will stave off a climate emergency.

Photograph: Anadolu Agency/Anadolu

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“I come from Sweden and back there its almost the same problem as here, as everywhere, that nothing is being done to stop an ecological crisis despite all the beautiful words and promises,” she told the crowd.

“We are now facing an existential crisis, the climate crisis and ecological crisis which have never been treated as crises before, they have been ignored for decades.

“And for way too long the politicians and the people in power have gotten away with not doing anything. We will make sure that politician’s will not get away with it for any longer.”

Her speech came amid police efforts to forcibly clear Extinction Rebellion protesters from Waterloo Bridge as the group debated whether to continue its campaign of mass civil disobedience. Police said on Sunday night they had cleared all the protesters from Parliament Square.

The London mayor, Sadiq Khan, said the disruption was “counter-productive” to the cause of climate change and was stretching resources so much it could damage police’s ability to fight violent crime.

Extinction Rebellion had earlier said it expected its supporters would be cleared out of the two sites occupied without permission as police prepared to evict them if they declined to leave voluntarily.

Last week, the group gained global coverage for the disruption its tactics of civil disobedience caused in central London. On Sunday, the organisers said they intended to change tack and would offer to vacate some sites in exchange for the mayor acting on some of their demands.

The Metropolitan police said they had made 963 arrests and charged 42 people. The force’s leader, Cressida Dick, said the group’s tactics, centred on peaceful direct action, had caused too much disruption.

On Saturday, Oxford Circus and Piccadilly Circus were returned to normal use, after complaints from businesses about the blocking of some of the capital’s key arteries.

On Sunday, activists rushed to Parliament Square, when police turned up in force to try to clear five roadblocks. Activists were using lock-on devices to hold the space, as well as gluing themselves to the ground and each other in order to slow down the police.

Activists said there were three people locked on trees in the square with more ready to go up. They promised attempts to evict them would be “spectacular” and could take police all night. However, by Sunday evening the police said the square had been cleared.

The stage-truck on Waterloo Bridge was finally removed by 5am on Sunday after police spent most of Saturday and well into the night removing protesters glued and locked on to it. Police spent hours using angle grinders to cut free the two protesters who had locked themselves down on the top of truck, before winching them down and carrying them into the back of waiting police vans.

By Sunday night they were moving the activists to one side of the carriageway, and arresting those who are refusing to move.

The Met has needed support from about 200 officers from other forces to deal with the protests, which have been peaceful.

Khan said 9,000 officers had been involved in policing the protest so far. He said: “I share the passion about tackling climate change of those protesting, and support the democratic right to peaceful and lawful protest, but this is now taking a real toll on our city – our communities, businesses and police. This is counter-productive to the cause and our city.”

The mayor added: “I remain in close contact with the Met commissioner, and agree that Londoners have suffered too much disruption and that the policing operation has been extremely challenging for our over-stretched and under-resourced police.

“I’m extremely concerned about the impact the protests are having on our ability to tackle issues like violent crime if they continue any longer. It simply isn’t right to put Londoners’ safety at risk like this.”

The protest group said a phalanx of police vans were gathered around Waterloo Bridge on Sunday amid mounting expectation protesters would be forced out.

Ronan McNern, a spokesperson for Extinction Rebellion, said: “We think they want everything cleared by the end of the week. People are willing to be arrested. There is a deep sense we do not want to be attached to any single site. What this disruption is doing, we are the news now. It is making people talk in pubs and buses about Extinction Rebellion. It makes them think about their existence which is under threat.”

Extinction Rebellion is discussing withdrawing from some sites in return for being allowed to remain in others and having its demands met.

She wrote: “Today marks a transition from week one, which focused on actions that were vision-holding but also caused mass ‘disruption’ across many dimensions (economic, cultural, emotional, social). Week two marks a new phase of rebellion focused on ‘negotiations’ where the focus will shift to our actual political demands.”

She continued: “We want to show that XR [Extinction Rebellion] is a cohesive long-term, global force, not some flash in the pan.”